1. Field of the Invention
The present invention pertains to a method of molding fluoropolymer and the molded article obtained using the method characterized by the fact that the molded article has excellent resistance to chemical solutions, excellent gas impermeability, and low linear expansion coefficient.
2. Description of Related Art
Fluoropolymers having excellent heat resistance and chemical resistance and other characteristics are used in manufacturing articles such as pipelines, tanks, and other lining materials, as well as chemical liquid transfer pipelines, joints, and chemical liquid storage containers, especially for use in semiconductor manufacturing processes and in chemical plants.
Among said fluoropolymers, polytetrafluoroethylene (PTFE) has the best heat resistance, chemical resistance, and other characteristics. PTFE is not melt processible because it has an extremely high melt viscosity (at least 108 Pa·s at 380° C.). Therefore, it cannot be molded by such conventional polymer processing methods as melt extrusion molding, injection molding, blow molding, transfer molding, melt compression molding, or other melt processing methods.
As a result, for PTFE lacking melt processibility, molding is performed by means of paste extrusion molding, compression molding, or other non-melt processing method. In the paste extrusion molding, shear is applied to a mixture of fine powder PTFE and lubricant (typically a hydrocarbon) to make a paste, which is extruded at a low temperature (lower than 75° C.). In compression molding, the granular PTFE powder, kept at a temperature higher than the crystalline transition point (about 19° C.), is charged to a casting mold for compression and then heated (sintered) to form the desired article.
However, in the paste extrusion molding method, after the extrusion, the lubricant must be removed. Residues of the lubricant remaining in the molding (molded article) can be carbonized, leading to contamination and discoloration of the molding, and deterioration in properties such as chemical resistance, electric characteristics. In addition, in removing the lubricant, it is necessary to raise the temperature gradually in order to prevent cracking due to too rapid volatilization (bumping) of the lubricant. This is undesirable.
In the case of compression molding, the molded article is limited to simple shapes. When PTFE moldings in complicated shapes are desired, mechanical processing (machining) has to be performed on blocks of PTFE formed by compression molding. This is a complex and expensive process.
An alternative to PTFE is tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA). It has heat resistance and chemical resistance as good as that of PTFE, but being melt processible, it can be processed by such conventional polymer processing methods as melt extrusion molding, blow molding, transfer molding, and melt compression molding. Therefore articles of PFA can be mass produced at a cost lower than comparable articles of PTFE. This is an advantage.
However, PFA has chemical liquid resistance and gas impermeability properties inferior to those of PTFE. As a result, it has been proposed that PTFE be blended in PFA so as to increase the crystallinity of the molding to improve the chemical liquid resistance and gas impermeability. However, the PTFE usually employed as a molding powder has a high molecular weight, so that as the quantity added to PFA is increased, the viscosity rises drastically, and melt processing becomes difficult to perform. This is undesirable. On the other hand, when such a composition with a higher viscosity is used to perform compression molding, paste extrusion molding, or other non-melt processing just as for PTFE, the shape is restricted, and productivity deteriorates significantly so that this scheme is not practical.
Japanese Kokai Patent Application No. 2002-167488 (equivalent: U.S. Pat. No. 6,649,699) and Japanese Kokai Patent Application No. 2003-327770 (equivalent: U.S. Pat. No. 7,030,191) propose a scheme in which a low-molecular weight PTFE is used to prevent rise in the viscosity so as to enable melt processing and to improve the chemical resistance and gas impermeability. However, for the method of adding low-molecular weight PTFE, the quantity added is limited, and this is undesirable.
In addition, for molded articles prepared by sintering at a temperature higher than the melting point, the linear expansion coefficient of the resulting article is higher than that of the other materials, and, when used at a high temperature, problems occur. For example, a pipe fixed between joints will bend, affecting the sealing of the joints, allowing leaks. This is undesirable. Because the linear expansion coefficient is inversely related to the crystallinity of the molding, it is preferred that the crystallinity (crystalline fraction) of the molding be as high as possible and the amorphous (noncrystalline) fraction of the polymer be kept low. However, although the crystallinity of the molding can be increased by slowly cooling after sintering, the amount of crystallinity lost in the sintering process cannot be completely restored to the fraction present in PTFE as-polymerized, i.e. before sintering. Therefore this method inevitably results in some deterioration in the chemical liquid resistance, gas impermeability and linear expansion coefficient.